It's from The Barefoot Contessa Cookbook, so I can't take credit for the recipe, but it's my favorite!

2 large yellow onions
4 tablespoons unsalted butter
1/4 cup vegetable oil
1/4 teaspoon ground cayenne pepper
1 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
4 ounces cream cheese, at room temperature
1/2 cup sour cream
1/2 cup good mayonnaise

Cut the onions in half, then slice them into 1/8-inch thick half-rounds.

Heat the butter and oil in a large saute pan over medium heat. Add the onions, cayenne, salt and pepper, and saute for 10 minutes.

Reduce the heat to medium-low and cook, stirring occasionally, for 20 more minutes until the onions are browned and caramelized. Allow the onions to cool. (HEATHER'S NOTE: I put the onions in a food processor before I combine them with the other ingredients so they aren't big long strings in the dip.)

Place the cream cheese, sour cream and mayonnaise in the bowl of an electric mixer fitted with the paddle attachment, and beat until smooth. Add the onions, and mix well.

Taste for seasonings. Serve at room temperature.